In recent years, the mulching of garden beds, soil areas and even pot plants has become an important part of garden care and maintenance. Mulch is applied to reduce weed growth, retain soil moisture, keep soil cooler on hot days and, when organic materials are used, to add nutrients to soil for better plant growth. Mulch also helps tie a garden together and hide bare earth.
When the practice of mulching become popular in Australian gardens in the 1980s and 90s, it was thought that the thicker the mulch layer the better. Ongoing studies of the link between mulch and soil moisture have shown there is an optimum thickness and particle size for mulch.
Following this research, modern advice is that chunky mulch gives the best results. Chunky mulch resembles the mulch found naturally occurring on forest floors. Fine mulch thickly applied can prevent water from reaching soil. It may also lead to the formation of water-repellent soil under mulch and can draw water from the soil. Weed seeds are also more likely to germinate in fine mulch.
Mulch containing particles above 20 to 30 millimetres allows good air circulation around plants and also allows water from rain or irrigation to easily reach the soil below. The best mulch has more than 80 percent of its particles larger than 50 millimetres and should be applied at no greater depth than 50 millimetres. Good options include bark flakes, composted bark and chopped lucerne or sugar cane.
Fine organic mulch can be beneficial. Current recommendations are to apply a fine layer of organic mulch, compost or a soil conditioner to soil and then cover with coarser mulch.
Importantly, fine mulch should be water repellent, rather than water retentive, to prevent removing moisture from the soil. Don’t add wetting agents to mulch.
The right mulch for growing needs
To get the mulch right, consider the natural environment where a plant grows, to best understand its need for mulch. This is particularly important for plants such as cactus or succulents from desert areas that grow without a great deal of surface organic matter. Rocks are the main and most effective mulch in desert regions so a gravel-based mulch is the best choice for these types of plants.
For other types of drought-tolerant plants that grow outside of desert regions, keep mulches to no more than 25 millimetres and use woodchip or other coarse organic mulch.
To get mulch right and have a healthy, thriving garden, follow these simple recommendations:
- The finer the mulch, the more thinly it should be spread.
- Coarse mulch works best (80 percent of the particles should be larger than 5 millimetres).
- Don’t add a wetting agent to mulch.
- The optimum depth for mulch is no greater than 50 millimetres.
- Water well before applying mulch or apply after rain.
- Check wood-based mulch regularly for signs of termite activity and don’t put wood mulch adjacent to buildings.
- Don’t put mulch around trunks or stems.